The present invention relates to an auto-focussing system to be employed in an optical type information record/reproduction system for recording/reproducing information to/from a disk-shaped optical type record carrier body.
One of the most important techniques when reading or writing information from or onto, respectively, an optical type record carrier body, such as an optical disc, is to suitably form a light beam spot on the information plane of the disc. In the optical type information record/reproduction (used throughout this specification as an abbreviation for "record and reproduction") system, a focussing servo unit is provided for allowing an objective lens to properly follow the surface oscillation of the information plane of the disc which is caused when the disc rotates, thereby controlling the light spot to be accurately formed on the information plane. The position of the optical system including this objective lens is controlled in the manner such that the distance between the information plane and this optical system is always set to a value within a proper range in dependence upon the surface oscillation of the disc. A search system unit is provided for automatically placing the objective lens within a focus pull-in (retractable) range of the above-mentioned servo unit.
However, with conventional techniques it is impossible to sufficiently and effectively perform the task of positioning the objective lens in order to focus the light spot on the disc information plane. For example, according to an optical type information record/reproduction apparatus as disclosed in a Japanese Patent Disclosure (KOKAI) No. 108939/80, when the objective lens approaches the rotating optical disc and enters the focus pull-in range of the servo unit, a control signal is generated from a predetermined circuit section to stop the movement of this lens. Thereafter the objective lens is pulled in by the servo unit.
In such a conventional arrangement, if the optical system malfunctions, even when the objective lens is positioned in the pull-in range, the circuit section still remains non-operative, whereby a lens-movement-stop signal will not be produced. The lens will continue to move toward the disc and an optimum focussing of the light spot is not achieved. In the worst case, the lens will come into contact with the rotating disc, causing damage to the hardware of the apparatus.
According to U.S. Pat. No. 4,368,526, a technique is disclosed whereby when a constant time passes after the focus servo loop for the optical disc had been closed and at the same time the servo open loop gain had been set into the full gain, the supply of a focus searching voltage to the servo system is stopped. According to this reference, the system is arranged such that a focus error signal is always monitored and the focus servo system is closed when the level of this signal exceeds a predetermined threshold level. However, even if this technique is used, there is still a danger such that, for example, when a malfunction occurs in the optical system, the objective lens will collide with the surface of the rotating disc.